Growth factors are substances, such as peptides, which affect the growth and differentiation of defined populations of cells in vivo or in vitro.
Bone formation occurs during development of long bones (endochondral bone formation) and flat bones (intramembraneous bone formation). Further, bone formation occurs during bone remodeling which occurs continuously in adult life in order to preserve the integrity of the skeleton. Finally, bone formation occurs during bone repair, such as when bone wounds occur in a fracture or surgical situation, for example. While separate bone formation mechanisms are thought to be involved in the embryological development of long and flat bones and repair is thought to involve intramembraneous bone formation.
Bone formation by either mechanism involves the activity of osteoblasts, which are regulated by growth factors. Osteoblasts are derived from a pool of marrow stromal cells (also known as mesenchymal stem cells; MSC). These cells are present in a variety of tissues and are prevalent in bone marrow stroma. MSC are pluripotent and can differentiate into a variety of cell types including osteoblasts, chondrocytes, fibroblasts, myocytes, and adipocytes. Growth factors are thought to impact osteogenic cell proliferation, differentiation and osteoblast mineralization, each of which impacts bone formation.
Autogenous bone has been used, such to repair bone in patients with craniosynostosis and cleft grafting, for example. Craniosynostosis (CS), the premature closure of cranial sutures, affects 1 in 3,000 infants and therefore is one of the most common human congenital craniofacial deformities. Premature suture closure results in cranial dimorphism, which may need surgical correction. Premature suture closure in human CS may occur by two possibly distinct processes: calvarial overgrowth and bony fusion. Recently, FGF2 and FGFR1 have been implicated in premature cranial suture fusion via CBFA1-mediated pathways (8). Missense mutation of CBFA1 is linked to cleidocranial dysplasia, manifested as delayed suture closure.
Autologous bone grafting procedures have been performed utilizing autogenous bone, such as from the iliac crest or calvaria. These donor sites are not without associated morbidity including pain, gait disturbance, thigh paresthesia for iliac crest donor sites, and infection, neurologic deficits, and hematomas for calvarial grafts. Further, donor sites may have limited volume and may contribute to increased surgical time and hospital stay.
Alloplastic grafting materials have also been utilized, and growth factors have been tested in animal models. For example, bFGF has shown potential for use in bone regeneration and repair. Another family of osteogenic growth factors have been described as bone morphogenic protein (BMP). Specifically, BMP-2 recombinant protein has been demonstrated to regenerate mandibular continuity defects and cleft palate defects with results equal to or better than autogenous particulate bone and marrow. BMPs and other osteogenic factors have been studied for use in clinical applications. However, the cost of using minimally effective dosages of BMP has been a limiting factor in clinical use.
Spinal fusion is a surgical technique in which one more of the vertebrae of the spine are united together so that motion no longer occurs between them. Indications include: treatment of a fractured (broken) vertebra, correction of deformity, elimination of pain from motion, treatment of instability, and treatment of some cervical disc herniations. The surgery may involve placement of a bone graft between the vertebrae to obtain a solid union between the vertebrae. The procedure also may involve supplemental treatments including the placement of plates, screws, cages, and recently bone morphogenic protein 2 and 7 to assist in stabilizing and healing the bone graft. Autogenous bone grafting has been the clinically preferred method, and yet has about a 30-50% failure rate. Autogenous bone grafting is a separate surgery and also carries significant morbidity.
Therefore, safe, effective and affordable compositions and methods are desired to induce bone formation in bone development, disorders, or bone trauma.